Apparatus for joining thermoplastic sheet material



Nov. 29, 1955 s. s. MlNl-:R ETAL 2,725,091

APPARATUS FOR JOINING THERMOPLASTIC SHEET MATERIAL Original Filed Oct. 19, 1949 5 Sheets-Sheet l INVENTOR` BY n : 1-Uff. s

ATTORNEY Nov. 29, 1955 s. s. MINER ETAL APPARATUS FOR JOINING THERMOPLASTIC SHEET MATERIAL 5 Sheets-Sheet 2 Original Filed Oct. 19, 1949 ATTORNEY Nov. 29, 1955 ss, yMlNER ETAL 2,725,091

APPARATUS FOR JOINING THERMOPLASTIC SHEET MATERIAL Original Filed Oct. 19, 1949 3 Sheets-Sheet 3 WHL? ATTORNEY Utlited States Parenti` @fr APPARATUS FOR JoINlNG THERMOPLASTIC SHEET n/rATERIAL Samuel S. Miner and Edi'vard Clark, Mishawaka, Ind., a'ssignors to United States Rubberk Company, New York, N. Y., a corporation of New `ersey Original application October 19, 1949, Serial No. 122,212. Diided and this application April 22, 1954, Serial No. 42 ,617

10 Claims. (Cl. 154-42) This invention relates to a method and apparatus for joiningithermoplastic sheet material,y particularly sheet polyethylene, especially of substantial thickness, i. e., of a thickness greater than that of foil, typically ranging from 0.01 inch upwardly to any desired thickness, say 0.5 inch. n

This application is a division of application SerialNo. 122,212, filed October 19, 1949, now Patent 2,698,273, wherein the method of this invention is claimed.

The problem of making satisfactory joints between sheets of polyethylene has existed for a considerable period of time. Polyethylene has a very sharp` melting point. lust below the melting point it is a solid, while just above it, it is a free-flowing liquid. Therefore, the usual type of heat-sealing devices, which are satisfactory for joining such thermoplastic materials as polyvinyl chloride or copolymers of vinyl chloride or vinyl acetate, which only gradually become free-tlowing liquids as the temperature is raised, will not work on polyethylene.

y, The cc-pending application of Romeyn et al., Serial No. 2,325, led January 14, 1948, now Patent No. 2,520,737,1assigned to the assignee of the present invention, describes what is believed to be the first successful method of joining -sheets of polyethylene ofsubstantial thickness. ln the method of that application, the joining is effected by a very careful control of temperature and pressure, specifically by heating the` material at the interface of the overlap to a temperature of not less than 5 C. below and not more 4than 15 f C. above the melting point while applying pressure at the overlap, the heating and application of pressure being continued :for a period of time substantially only that required to produce a satisfactory joint. n i v `The presentinvention provides a method and apparatus for the joining of polyethylene sheets of substantial thickness without the precise control of temperature and pressure required by the above-mentioned application. i i

While the invention is` especially/adapted to joining polyethylene sheets, the joining of which presents a particularly diflicult problem, it may if desired be employed toetfect homogeneous and integral joining of sheets of other thermoplastic materials which ldo no t `present the Adifficulties attendant upon the successful joining of poly- Iethylene sheets. Examples ofksuch other ,thermoplastic materials which may be joined vby following the principles of the present invention are: polyvinylpchloride, copolymers of vinyl chloride and vinyl acetate, polyvinyl acetals, e. g., polyvinyl butyral, polyvinylideneuchloride, polystyrene, acrylic resins such as polymeric methyl acrylate, polymeric methyl methacrylate and similar polymeric esters of acrylic acid and methacrylic acid,`c'ellu`lose acetate, ethyl cellulose, etc. 4 i

In splicing sheets of thermoplastic material, the seam area where the sheets are lapped yto form the joint is twice as thick as the single sheet of thermoplastic 'inaterial, and,ftherefore, twice as much heat imust be lsupplied as would be required 'to soften a single layer of 2,725,091 Patented Nov.` 29, 1955 2 the material. Consequently, the areas adjacent to the splice become veryhvot and soften much in advance of the lapped splice itself. For this reason splices in thermoplastics, especially those having a sharp melting point and low thermal conductivity like polyethylene, are very diicult to make. The present invention, however, overcomes this difficulty and makes it possible to eifect such splices in a simple and commercially feasible manner.

The present invention is particularly advantageous because it enables the joining of polyethylene sheet materials so as to form an integral homogeneous one piece container of any desired size. Such containers are of extreme value for the storage vandl transportation of corrosive chemicals and for this rea-son the present invention is especially important since it provides a method of making such containers with ease and facility.

In the drawing:

Fig. 1 is a top plan View of a device, constructed in accordance with the invention, for making a lap joint between two at sheets of thermoplastic material.

Fig. 2 is a longitudinal section on the line 2-2 of Fig. l; the appearance of the platen-carrying portion of the device when open is portrayed in dotted lines.

Fig. 3 is a transverse section on the line 3 3 of Fig. l.

Fig. 4 is a side view, with the joint-forming portion in longitudinal section, of a unit for making endless circular` joints; this unit is especially adapted for joining the 'domed ends of a chemically resistant thermoplastic tank 'to a cylindrical body portion; it can also be used vto join two cylindrical body portions to form a longer cylindrical body.

Fig. 5 is a front view of the joint-forming portion of 'the unit of Fig. 4, taken on the line 5-5 of Fig. 4.

Fig. 6 is a section on the line i6`"6 of Fig. 5.

Fig. 7 is a vertical sectional view 'of a unit for joining a ilanged collar toa flange projecting'y from the body of a cell.

The present invention is based upon the discovery that thermoplastic sheet materials, especially polyethylene in sheet form' at least 0.01 inch thick, 4may be integrally and homogeneously joined to form a joint which is just as strong as the remainder of the assembly, by lapping `two surfaces of the sheet material', heating the material in the area of the joint `to a temperature substantially above its softening point and thereby causing it to fuse yand ow together at the site of he joint, simultaneously maintaining the areas of the material immediately adjacent to and on each 'side of the area of 'the join't at a temperature substantially below the softening point by cooling, and coniining'the material at the joint "under uniform pressure as it is being melted', while it is in molten condition, and thereafter during the subsecfpi'ent cooling step until the material has cooled to a temperatnre substantially below its softening point. After uthe joint has been etie'cted in this manner, heating is discontinued and the 'joint is cooled to a temperature substantially ybelow its melting point in order to slidify the material forming the joint (which was melted and caused to flow together underl the heat and pressure)l and permit release of pressure and removal of the joint from the equipment.

In the preferred practice of "the invention, uniform fluid pressure is applied to 'the material in the area of the joint and to the cooled adjacent "areas by means of a 'iiekible iniiatablepressure-resisting bag which is inflated by a gas under pressure, usually'air. i

The invention yis based upon'the discovery 'that vwhen the thermoplastic sheet material at thev joint is heatedup to above the softening point so that it becomes liquid, the" normal Atendency of the thermoplastic material "to flow out under the pressure*exerted'jat the joint may be prevented yin a simple and veconilical way by Acooling at each side of the joint so that the hot flowing thermoplastic material is cooled quickly, causing it to harden up at the edges of the joint. If these edges are not cooled in this way the molten material at the joint would ow out, under the pressure exerted, to such an extent that there would be practically no material left at the site of the joint. In the invention described in the prior application of Romeyn et al., mentioned above, it was necessary to apply both heat and pressure judiciously. With the present invention the problem of holding the temperature and pressure `within critical limits is eliminated, since in the present invention it does not matter how fluid the material in the area of the joint becomes or how much pressure is exerted upon this molten material because the provision of cooling7 on each side of the joint and the exertion of pressure across the lines between the joint and the cooled areas prevents the escape of the molten thermoplastic. The cooled material along the edges of the joint serves completely to conne the molten material between the boundaries of the heating platen, the pressure bag and the lateral cooled portions. Thus there are no mechanical confining means along the lateral edges of the joint, the material itself serving as the lateral barriers.

The drawings portray three different embodiments of apparatus for carrying out the present invention. These different embodiments are especially designed for effecting particular types of joints.

Referring now to the embodiments portrayed in Figs. l to 3 in the drawings, there is provided a lower stationary platen 1 ixedly supported in any suitable manner on support 2 and an upper platen member 3 which is pivoted at one end at 4 to one end of lower platen 1. Platen-carrying member 3 ixedly carries on its lower face a heating platen 5, heated by electrical resistance element 6, and two cooling platens 7 and 8 on each side of heating platen 5. Cooling platens 7 and 8 are cooled by the circulation of a coolant liquid through the pipes 9, the coolant entering and leaving via flexible tubes 10 and 11. The heating platen and the cooling platens 7 and 8 are insulated from each other by insulation 12 and from the supporting member 3 by insulation 13.

The lower platen 1 is hollowed out to receive a rubber bag 14 equipped with valve 15 and supplied with inflating gas (almost invariably air) under pressure by flexible tube 16 for the purpose of applying pressure across the area of the joint and the adjacent cooled areas.

The upper platen member 3 is provided at its free end with a. hook 17 which is adapted to lock the upper and lower members 3 and 1 together to resist the pressure exerted by the bag 14 while a joint is being formed. When thus locked, the upper platen is held stationary with respect to the lower platen.

In operation, two pieces of thermoplastic sheet material, 18 and 19 are laid across bag 14 while the press is open. The press is then closed, bag 14 is inflated, and platen 5 is heated to above its softening point while platens 7 and 8 are cooled by the circulation of coolant through pipes 9 to hold the material at the sides of platen 5 at a temperature well below the softening point. After the seam has been made, the heating of platen 5 is discontinued until the joint has been cooled to a temperature well below the softening point after which the pressure is released and the press opened.

In the embodiment portrayed in Figs. 4 to 6 a circular seam is shown as being made between a cylindrical body portion 20 and a domed end cap 21 having a fitting opening 22, by a unit which utilizes the principles described above but which is especially adapted to the formation of circular joints. The unit comprises an annular ring or wheel 23 comprising three segments held together by pins 24. By removal of pins 24, the ring 23 can be collapsed inside the formed container, the three segments of ring 23 being removable through the relatively small titting opening 22. An oblique break 23a is provided be- 4 tween two of the three segments of ring 23 in order to permit assembly and dis-assembly of the wheel 23.

Ring 23 is held in position by the three radial rods 25 which are pivoted at their inner ends to the enlarged end of a sleeve 26 so that rods 25 are moved from the radial position to the horizontal position when sleeve 26 is moved rightwardly of Fig. 4 along stationary staft Z7. The position of the rods 25 when they are thus unlocked from supporting engagement with wheel 23 is shown in dotted lines in Fig. 4. It will be understood that the outer ends of rods 25 are rounded slightly and engage slightly rounded depressions in lugs 2S which are integrally attached to the inside faces of the sections of the wheel 23. Thus the rods 25 can be moved into and out of supporting engagement with wheel 23 by the longitudinal movement of sleeve 26 which may be effected by a crank 29 actuated by a rod 30 which may be connected to a foot treadle (not shown). A safety latch 31 may be provided to prevent accidental movement of sleeve 26.

The ring 23 has on its outer face an annular channel in which is disposed the annular inflatable rubber pressure bag 32 supplied with air under pressure by tube 33 and the usual valve.

Surrounding ring 23 is an external ring member 34 disposed exteriorly of the portions 20 and 21 being joined. Ring 34 carries the heating platen 35 and the cooling platens 36 on each side thereof and separated therefrom by insulation 37.

Ring 34 is supported by an eye member 38 through which passes horizontal support 39. Support 39 can be adjusted by means of turnbuckle 40, so as to center ring 34 with respect to wheel 23. Notch 39a is formed in the upper face of support 39 to receive eye 38 and prevent undesired longitudinal movement of ring 34 out of alignment with wheel 23.

Platens 36 are cooled by coolant liquid supplied and withdrawn by flexible tubes 41 and 42, and circulated through annular cooling tubes 43 in platens 36. Platen 35 is heated by electrical resistance element 44.

Arm 39 and shaft 27 are supported by vertical support 45.

The operation of the unit of Figs. 4 to 6, when making a cylindrical thermoplastic vessel having domed ends, is as follows. The equipment and sections 20 and 21 are first arranged as shown in Fig. 4, the end cap 21 having opening 22 being applied before the other end cap (not shown but which has no opening), to pei-mit assembly. Bag 32 is inflated and platens 3S and 36 are heated and cooled, respectively, to effect the joining. Application of heat to platen 35 is then discontinued and the joint is then cooled down to well below the softening point. Bag 32 is then deflated and spokes 25 are retracted to the dotted position of Fig. 4 by releasing latch 31 and pulling down on rod 30. Then the assembly comprising joined sections 20 and 21 and the wheel 23 is removed and reversed from the position shown in Fig. 4, the opening 22 now surrounding sleeve 26. Wheel 23 is now moved into longitudinal alignment with platen-carrying ring 34 and with the overlapping portions of section 20 and the other end section (not shown, but which is now in the position formerly occupied by section 21) which are to be joined. Spokes 25 are then moved into radial supporting engagement with wheel 23. The air bag 32 is then inflated to force the material at and adjacent the joint against the platens 35 and 36. The joint is next effected. After the joint has been made and cooled down to below the softening points, the spokes 25 are retracted again to the dotted-line positions, bag 32 is deflated, and the assembly including the completed container with wheel 23 inside it is removed from the machine and taken to the bench where pins 24 are withdrawn and the three sections of the collapsible wheel 23 are removed through opening 22.

It is often preferred to lay a sheet of non-thermoplastic foil, preferably one of aluminum because of its excellent heat-transmitting characteristics, across the sectional platens of the units portrayed in Figs. 1 t0 6, in order to prevent the insulation 12 and 37 between the sections from leaving an impression in the splice.

Referring now to the embodiments of Fig. 7, this is employed to weld flange 50 of an angular collar 51 to ange 52 projecting outwardly from a depressed portion S3 of a body S4. Flange 50 and ange 51 are overlapped and placed in the press as shown. The annular rubber inflation bag 55 is square in cross-section and when inflated applies pressure not only to the overlapped area but to the horizontal portions of the collar 51 and of the depressed portion 53,as well. Inflation air is supplied to bag 55 by tube 55a which enters through ring 60. The annular heating platen 56, heated by element 57, is adjacent to the overlap in vertical position and thertwo cooling platens 58 and 59 are positioned at right angles to platen 56 so as to cool any material which would tend to ilow out from the splice area. Ring 60, surrounding bag 55 and upper cooling platen 58, restrains the bag and enables the bag when inated to exert pressure on the joint and on the adjacent cooled areas. Ring 60 also functions as a spacer to limit the `application of mechanical pressure to the press. Ring 60 is so arranged as to admit the material 53 between it and the cooling platen 59. Cooling platen 59 is fixedly carried on a bottom support 61 while cooling` platen 58 is xedly carried on an upper pressure-applying member 62. Pressure is applied to element 62 by hand wheel 63 which screw-threadedly engages centralpost 64 which is fixedly attached as by welding 65 to bottom member 61.

The operation of the embodiment of Fig. 7 will be obvious from the foregoing but may be briey summarized as follows: v l

The member 61 is first positioned inside the body 54, which should be open at one end, with the portion 53 along cooling platen 5*) as shown. Then ring 69, carrying bag 55, and ring-like heating platen 56 are placed into the position shown. Then the flanged collar 51 is inserted after which the pressure-applying element 62 is placed into position over post 6ft. Pressure is then applied by means of handwheel 63. Then bag 55 is inflated and Athe cooling platens 5S and 59 are cooled and heating platen 56 is heated to effect the joint. Thereafter operation is as before, element 6l being finally removed from within the body 5ft through an open end thereof. lt will be obvious that one or both of the cooling platens 58 and 59 may be disposed at an angle other than ya right angle to the heating platen 56, when it is desired to form a shapeother than that portrayed in Fig. 7.

lt will also be obvious that if desired the cooling platens 7 and S (Figs. 1 to 3) Sii and 59 Fig. 7) may be grooved on the side next to the cooling tubes in a manner similar to that shown in Fig. 6 in order Ato assure intimate Contact and good heat conductivity between the cooling tubes and the cooling platens.

Typical technique withA polye'tltyene ln a typical'example of making a splice from sheets of commercial polyethylene, 0.1 in. in thickness, pressure of about 5 lbs/sq. in. is rst applied to the air bag. The the heating element is turned on. r[he air pressure is then increased to about l0 lbs/sq. in. as the splice heats up. I his serves to squeeze out trapped air. Cooling water is introduced into the side sections of the unit shortly after the center section has started to heat. About 5 minutes elapse from the time the heating element is turned on until the polyethylene at the splice is completely fused and flowed together. Then the power in the heating element is cut off and the rate at which water flows through the cooling elements is increased in order to cool down the entire splice as rapidly as possible. This requires about 6 minutes. The temperature of the splice during fusion is between 250 and 350 F, The area beneath the cooled sections of the platen is maintained at about 150 to 180 '3 F. the splice is being effected. After the splice has been ,completed it is cooled to below approximately F. before it is rerriove'd.

General The cycle given for commercial polyethylene in the general preceding paragraph will vary where other thermoplastics are used. For instance, when sheets of a mixture of polyisobutylene (e. g., Vistanex) and polyethylene are to be spliced, a higher temperature must be used since such a mixture softens at a higher temperature than polyethylene. The time and temperature will also vary with the type of splice. For instance, the circular splice takes a longer time than a straight line splice because there is more mass ,toy heat up in the ring platen. Of course, all these variations can be made by the operator according to the particular needs. Those skilled in the art will be enabled to effect any desired splice by following ther teachings of this specification. n

In general, the heating platen is heated to a temperature substantially above the softening point, i. e., sufficiently thereabove to make the material completely molten so that it iiows together to form a perfectly unbroken joint. It is preferred to heat the material at the joint to at least 10 F. above the softening or melting point, typically to from 10 to 110 F. thereabove.

In general, the cooling platens are cooled to such an eXtcnt as to maintain the material at the sides of the splice at a temperature substantiallyv below the softening point, i. e., suiiicientlytherebelow that the material cannot ilow under the pressure exertedby the bag. it is prefered to cool to such an extent as to keep the temperature of the material at the sides of the joint at least as low as 50 F. below the softening or melting point, commonly to from 50 to 100 below `the melting point.

ln general, the joint is finally cooled to at least 50 F. below the melting point and'oftento at least 130 F. therebelow before releasing the pressure.

As indicated above, in thecase of commercial polyethylene, having armelting point of C. (239 F.), it is often preferred to heatthe joint to 250 to 350 P., to cool the adjacentareas to to 180, and to finally cool the joint to below *100 F. before `removing it.

ln general the pressure applied by the bag should be suflicient to cause the molten thermoplastic to flow together to give a unitary joint and to restrain and confine the molten materiallat the splice until it has solidified to such extent' as to be incapable of owing under the pressure applied. Usually the pressure applied during splicing will be within the range of from l0 to 100 lbs/sq. in.

By the present invention heat, cooling and pressure are applied so as to cooperate in anew way to produce results never before achieved. Thesimultaneous melting at the joint, cooling of adjacent areas and application of uniform uid pressure across the joint by means of the inflated rubber bag so cooperate that a perfectly homogeneous joint is produced in a simple and commercially feasible manner.

Having thus describedv my invention, what l claim and desire to protect by Letters Patent is:

l. Apparatus for joining lapped thermoplastic sheet material comprising arigid heating platen, means for heating said platen to such an extent as to raise the sheet material at the joint to a temperature above its softening point, rigid "cooling platens located on each side of said heating platen for cooling the areas of the sheet material imme diately adjacent the joint to lbelow its softening point, a flexible inflatable pressure-resisting bag extending across said heating and'coo'ling platens, said platens and said bag being so arranged that lapped sheets to be joined are receivable therebetween, with the lapped portions opposite said heating platen, and means for'holding said platens and said bag in such manner that when said bag is inflated it presses the sheets to be joined against said platens.

2. Apparatus for joining lapped thermoplastic vsheet material comprising a rigid heating platen, means for heating said platen to such an extent as to raise the sheet material at the joint to a temperature above its softening point, rigid cooling platens located on each side of and in alignment with said heating platen for cooling the areas of the sheet material immediately adjacent the joint to below its softening point, a flexible inflatable pressure-resisting bag extending across said heating and cooling platens, said platens and said bag being so arranged that lapped sheets to be joined are receivable therebetween with the lapped portions opposite said heating platen, and means for holding said platens and said bag in such manner that when said bag is inflated it presses the sheets to be joined against said platens.

3. Apparatus for forming an endless ring-like joint between two lapped portions of thermoplastic sheet material comprising a rigid annular heating platen, means for heating said platen to such an extent as to raise the sheet material at the joint to a temperature above its softening point, rigid annular cooling platens located on each side of said heating platen for cooling the areas of the sheet material immediately adjacent the joint to below its softening point, a Flexible inflatable pressure-resisting bag concentric with and extending across said heating and cooling platens, said platens and bag being so arranged that the lapped portions of sheet material to be joined are receivable therebetween with the lapped portions opposite said heating platen, and means for holding said platens and said bag with reference to each other in such manner that when said bag is inflated it presses the portions of sheet material to be joined against said platens.

4. Apparatus for forming an endless ring-like joint between two lapped portions of thermoplastic sheet material comprising a rigid annular' supporting member adapted to be disposed inside the joint, a flexible inflatable pressure-resisting bag surrounding and supported by said annular member, a rigid annular heating platen disposed outwardly of said bag, means for heating said platen to such an extent as to raise the sheet material at the joint to a temperature above its softening point, and rigid annular cooling platens on each side of said heating platen for cooling the areas of the sheet material immediately adjacent the joint below its softening point, said bag extending across said heating and cooling platens, said platens and said bag being so arranged that the portions of sheet material to be joined are receivable therebetween with the lapped portions opposite said heating platen.

5. Apparatus as set forth in claim 4 wherein said annular supporting member is collapsible to permit its removal through a relatively small opening in the wall of a container formed with the apparatus.

6. Apparatus as set forth in claim 4 wherein said annular supporting member is collapsible to permit its removal through a relatively small opening in the wall of a container formed with the apparatus and further embodying :t stationary shaft co-axial with said annular supporting member, an axially movable sleeve around said shaft, rods pivoted on said sleeve and adapted to extend radially into supporting engagement with said annular supporting member, and means for causing movement of said sleeve along said shaft to move said rods from radial supporting positions to positions parallel to and closely adjacent to the axis of said shaft and vice versa, whereby a one piece cylindrical container having ends integrally welded to a cylindrical body portion with only a single relatively small opening in its wall may be formed in said apparatus.

7. Apparatus for integrally uniting a flanged collar of thermoplastic sheet material to an annular flange projecting from a body of said thermoplastic sheet material comprising a rigid annular outer member adapted to surround the joint, a rigid annular heating platen located within said member, means for heating said platen to a temperature above the softening point of the material, a flexible inflatable pressure-resisting bag surrounding said heating platen and disposed between said outer member and said heating platen, rigid annular cooling platens at an angle to said heating platen and located on each side thereof, said platens and said outer member being so arranged as to receive the two flanges to be joined in overlapped relationship with the lapped portions opposite said heating platen, said bag extending across said heating and cooling platens, and means for holding said cooling platens against the pressure of said bag so as to cause said bag to press the sections to be joined against said heating platen at the joint and against said cooling platens at the sides of the joint.

8. Apparatus for integrally joining thermoplastic sheet material which comprises rigid means for heating the overlapped sheet material in the area of the joint to a temperature substantially above its softening point and thereby causing it to fuse and flow together to form an integral joint, rigid means on each side of said heating means for simultaneously cooling the material immediately adjacent to the area of the joint to a temperature substantially below its softening point, means for subsequently cooling the resulting joint to a temperature substantially below its softening point, and fluid pressure means for uniformly pressing said material in the area of the joint against said heating means and the material immediately adjacent to the area of the joint against said rigid cooling means while a joint is being produced, whereby the molten material at the joint is conned under uniform pressure by said heating means, said fluid pressure means and the cooled material on both sides of the joint during the application of heat by said rigid heating means and during the subsequent cooling.

9. Apparatus for integrally joining sheet thermoplastic material which comprises a first rigid platen, second and third rigid platens on each side of said first named platen, means for heating said first named platen during the production of a joint and for subsequently cooling said tirsV named platen to solidify a joint after it has been produced, means for cooling said second named and third named platens, a flexible, inflatable, pressure-resisting diaphragm extending across said first named platen and across a substantial portion of each of said second named and third named platens adjacent the sides of said first named platen for an extent sufficient to form with the unmelted sheet material opposite said second named and hird named platens an effective seal preventing escape of molten thermoplastic material from the area of the joint, rigid means for supporting said diaphragm arranged so that sheets of material to be joined in overlapping relationship are receivable between said diaphragm and said platens and so that when said diaphragm is inflated it will press said material both at the joint and over a substantial distance at the sides thereof against said platens, and means for supplying an inflating fluid to the interior of said diaphragm.

l0. Apparatus for joining lapped thermoplastic sheet material comprising rigid means for heating the overlapping areas of said sheet material to a temperature above its softening point, rigid means for cooling the immediately adjacent areas of said sheet material on each side of said heating means to a temperature substantially below its softening point, and an inflatable, pressure-resisting bag extending across said heating and cooling means and adapted when inflated to press said sheet material against said heating and cooling means, and means for supplying an inflating gas under pressure to said bag.

References Cited in the file of this patent UNITED STATES PATENTS 2,524,932 Schulman Oct. 10, 1950 2,646,105 Langer July 23, 1953 FOREIGN PATENTS 549,803 Great Britain Dec. S, i942 

